Thermal Storage Devices Research Articles

Experimental investigation on the heat transfer performance of a latent thermal energy storage device based on flat miniature heat pipe arrays

An experimental latent-thermal energy storage device (LTESD) with a flat miniature heat pipe array (FMHPA) as a core heat transfer element is designed. Multihole flat pipes are utilized as a heat supply and heat removal loop for the passage of heat transfer fluid (HTF). Experiments are performed at different HTF volume flow rates and inlet temperatures to investigate the performance of the thermal storage unit consisting of FMHPA and vertical fins and to observe the change in the temperature of phase-change materials, such as lauric acid. The effects of heating/cooling section length and thermal resistance are also examined. Results indicate that LTESD works stably and efficiently and the respective storing and releasing power are 1299 and 1120 W under standard operating conditions of 2 L/min at 70 °C and 2 L/min at 15 °C.

Numerical simulation on thermal characteristics of supercooled salt hydrate PCM for energy storage: Multiphase model

Multiphase model is set up and applied to perform a 3D transient CFD simulation on the discharging characteristics of supercooled PCM. Three typical processes are identified and analyzed, i.e. stable supercooling, triggering crystallization and regular solidification periods. The simulation is achieved by loading the source term in energy equation and phase transition term in volume fraction equation. PCM temperature, solid fraction (SF) and heat transfer rate (HTR) to the surroundings are monitored and analyzed during the three stages. It is found that there is little difference among the temperature and SF curves of different z-direction planes during the first two periods but they are distinct during the regular solidification period. For the triggering crystallization period, PCM temperature rises much rapidly up to the melting point within 30s and SF increases up to 0.33 correspondingly. The heat loss is small in this short period and could be simplified as adiabatic. Variations of temperature and SF extend from up and side walls to the interior of the PCM unit. HTR curve fluctuates greatly throughout the three processes with highest value of 1420W occurring in regular solidification period. The developed method can be used for parameter analysis of supercooled PCM thermal storage devices.

Modeling and optimal design of thermal storage devices based on effectiveness-NTU approach and exergy recovery maximization

Modeling and optimal design of thermal storage devices based on effectiveness-NTU approach and exergy recovery maximization

Transient multi-day simulations of thermal storage and heat extraction for a finned solar thermal storage device

The effect of combining metal fins (Al) for heat spreading and recovery with phase change material (a mixture of NaNO3 and KNO3) on the performance of a thermal storage device is investigated. High-resolution transient simulations are performed covering two days of solar energy influx and heat extraction. The solver uses the enthalpy method to track melting, a strongly coupled implicit scheme to calculate conjugate heat transfer, and a dynamically refined mesh to ease grid creation and maximize computational efficiency. A potential application is thermal storage for solar cooking, although other applications can also be envisaged. The energy inputs of the simulations are based average solar radiation during a 48h solar cycle in New Delhi, India in June with a 1.5m2 solar reflector. Four different fin designs for an insulated latent heat thermal storage device (TSD) to be used with a solar cooker are tested. The four designs are compared based on their ability to spread heat evenly and rapidly into the phase change material (PCM) and the ease with which heat can be withdrawn from the device for cooking. The tests demonstrate the potential for using long term, multiday numerical simulations in the evaluation of TSD designs.

Numerical Analysis of Phase Change Material Characteristics Used in a Thermal Energy Storage Device

ABSTRACTIn this study, a numerical analysis is performed to investigate the freezing process of phase change materials (PCM) in a predesigned thermal energy storage (TES) device. This TES device is integrated with a milk storage cooling cycle operating under predefined practical conditions. Using this cooling unit, 100 litres of milk is kept cool at 4°C for 48 hours before it is collected. A 2-D model of the TES device is developed in COMSOL Multiphysics to analyze the phase change performance of water-based PCMs. The variations of thermal properties with temperature during the phase change are considered in the analysis. The model is used for exploring the solidification process of PCMs inside the TES device. Temperature variations with time, ice formation, and the impacts of boundary conditions are investigated in detail. Water PCM shows better characteristics in the solidification process in comparison to eutectic PCMs, which is mainly due to the differences between phase change temperatures of the PCMs.

Numerical Evaluation on a Direct-contact Thermal Energy Storage System

This study evaluates numerically various configurations of direct-contact PCM thermal energy storage devices, regarding inlet location, inlet flow directions, pre-heating and inlet tubes with straight fins. The direct-contact conjugate heat transfer between the heat transfer fluid (HTF) and PCM during melting process, is solved numerically by enthalpy-porosity formulation in the computational fluid dynamic approach. The results indicate that pre-heating could form channels in a short time, which improve heat transfer rate for charging stages. To further enhance heat transfer performance, inlet tubes embedded with straight fins. Compared to pre-heating method, the electric power can be saved. Each design are evaluated with respect to their heat transfer performance vis-à-vis heat storage ratio.

Generalized Heat Flow Model of a Forced Air Electric Thermal Storage Heater Core

Electric thermal storage (ETS) devices can be used for grid demand load-leveling and off-peak domestic space heating (DSH). A high-resolution three-dimensional finite element model of a forced air ETS heater core is developed and employed to create a general charge/discharge model. The effects of thermal gradients, air flow characteristics, material properties, and core geometry are simulated. A simplified general stove discharge model with a single time constant is presented based on the results of the numerical simulations. This simplified model may be used to stimulate economic/performance case studies for cold climate communities interested in distributed thermal energy storage.

Analysis of the effects of use of thermal energy storage device (TESD) in solar air heater

In this research work a setup is designed to incorporate Lauric acid as a phase changing material (PCM) in a solar air heater. This phase changing material (PCM) is carefully selected based on the application and manufacturing constraints. A thermal energy storage device (TESD) is manufactured and incorporated in solar air heater and experiments were carried out to compare the solar air heater with and without thermal energy storage device (TESD) to evaluate various parameters such as output temperature and pressure drop. Results revealed that the rise in the output air temperature decreases from 8.67K to 4K with the increase in the mass flow rate of air flowing through the solar air heater from 0.021kg/s to 0.035kg/s. Also with an increase in the mass flow rate, there is a decrease in the friction factor from 0.0119 to 0.00802. There is an average increment of 86.47% in the rise of output air temperature with TESD as compared to rise of the output air temperature without TESD with an average increment of 36.47% in friction factor. A computational analysis is also performed which gives insight into the working of solar air heater with thermal energy storage device (TESD). A computational domain of solar air heater with TESD is analyzed in CFD code FLUENT using various turbulence models such as k-ω SST, k-ω Standard, k-∊ Standard and k-∊ RNG. It is observed that results obtained from turbulence model k-є RNG model are in good agreement with the experimental results and hence used for the analysis of all the cases considered in this work.

Emission analysis of a CI engine during cold weather conditions using pre-heated air and engine using waste energy and phase-change material

ABSTRACTEnvironmental pollution due to emissions from an IC engine is an important problem appearing under cold weather conditions. In this study, air pre-heating as well as pre-heating of the IC engine is done to ensure efficient combustion and reduction in emission level during a cold start. A thermal energy storage device (TESD) and an air pre-heater is designed and tested for the purpose of pre-heating. The TESD contains paraffin as a phase-changing material which works on the principle of absorption and rejection of thermal energy during the phase-changing temperature Normal, discharge and pre-heat emissions for diesel and different blends of Karanja oil are compared. From the experiment, it is observed that carbon monoxide (CO), hydrocarbon (HC) and smoke emissions are reduced during engine pre-heating and air pre-heating compared to normal condition with respect to increase in time and load at compression ratio 18.

Thermal performance of solar air collection-storage system with phase change material based on flat micro-heat pipe arrays

In this study, a new type of solar air collection-storage thermal system (ACSTS) with phase change material (PCM) is designed using flat micro-heat pipe arrays (FMHPA) as the heat transfer core element. The solar air collector comprises FMHPA and vacuum tubes. The latent thermal storage device (LTSD) utilizes lauric acid, which is a type of fatty acid, as PCM. The experiments test the performance of collector efficiency and charging and discharging time of thermal storage device through different air volume flow rates. After a range of tests, high air volume flow rate is concluded to contribute to high collector efficiency and short charging and discharging time and enhance instantaneous heat transfer, whereas an air volume flow rate of 60m3/h during discharging provides a steady outlet temperature. The cumulative heat transfer during discharging is between 4210 and 4300kJ.

Thermoelectric cooling in combination with photovoltaics and thermal energy storage

The article deals with the use of modern technologies that can improve the thermal comfort in buildings. The article describes the usage of thermal energy storage device based on the phase change material (PCM). The technology improves the thermal capacity of the building and it is possible to use it for active heating and cooling. It is designed as a “green technology” so it is able to use renewable energy sources, e.g., photovoltaic panels, solar thermal collectors, and heat pump. Moreover, an interesting possibility is the ability to use thermal energy storage in combination with a photovoltaic system and thermoelectric coolers. In the research, there were made measurements of the different operating modes and the results are presented in the text.

Advances and prospects in thermal energy storage: A critical review

Thermal energy storage is an indispensible technology for adjusting the instability and time discrepancy between supply and demand of energy. It is mainly utilized for intermittent occasion, such as solar energy, variable energy load, and excessive energy that would be wasted rather than effectively utilized if it cannot be stored. Due to their potential for efficiently improving the comprehensive utilization rate of energy, thermal energy storage systems are of growing importance within the energy awareness: solar thermal utilization, peak shaving, industrial energy saving and waste heat recovery, building energy conservation, energy internet construction and so on. Developing efficient and inexpensive thermal energy storage devices is as important as developing new sources of energy, thus, in the past several years the thermal energy storage techniques have attracted a great deal of attention from both fundamental research and technological applications. Usually, there are three different mechanisms for thermal energy storage techniques: the sensible heat storage, the latent heat storage and the themochemical heat storage. The scope of this review is to give an overview and further analysis on the research which has been done on these three kinds of thermal energy storage techniques, and this review will be beneficial for the researchers and engineers to develop more efficient and optimized thermal energy storage systems. On the one hand, the research trends of three kinds of thermal energy storage techniques from 2000 to 2015 are carefully analyzed in the present review by using the statistics functions within the Web of Science Platform managed by Thomson Reuters. The statistics results show that according to the trends on paper numbers of each thermal energy storage techniques within the whole statistics period, the latent heat storage currently can be concluded as the most popular thermal energy storage technique in terms of fundamental research, the sensible heat storage is classified as least concern because main sensible heat storage systems so far have been almost matured, and the thermochemical heat storage is attracting more and more attention from researchers. On the other hand, the main technical characteristics (such as, energy storage density, energy storage scale, energy storage period, energy storage cost, advantages of technology, disadvantages of technology, future research focus, technology maturity and so on) of three kinds of thermal energy storage techniques are systematically compared in the present review on the basis of summarizing the previous related research work, and the final compared results reveal that because the key features for each thermal energy storage technology are totally different, the selection and extension of application fields for one thermal energy storage technique cannot move forward without a full consideration of its own technical characteristics. Finally, the latest progress on the development of thermal energy storage technologies is further discussed. The technical principles of the recently developed novel thermal energy storage concepts, mainly including of new latent heat storage materials represented by heat-storage ceramics and ionic liquids, the calcium based high temperature thermochemical heat storage systems and hybrid thermal energy storage systems, are introduced in detail, moreover, the technical potential and future outlook of these new thermal energy storage concepts are also addressed.

A PCM Thermal Storage for Ground-source Heat Pumps: Simulating the System Performance via CFD Approach

Abstract The conventional design of ground source heat pumps (GSHPs) is based on the peak heating and cooling loads. A possible optimization in GSHP design, including a thermal storage device between the ground exchangers and the heat pump, was already realized and it was found that a reduced-size geothermal field (-66%) is still able to cover the energy demand. In this paper, the design of the prototype was used as a starting point to study the potentialities of two possible upgrades for the optimization of the energy performance (COP) of the system. In the first case, the thermal storing material is water, as in the working prototype, and the efficiency is improved removing the cylindrical heat exchangers that were designed to separate the ground side from the heat pump side. In the second case, a completely new and more compact thermal storage was designed using phase change materials (PCMs). Computational fluid dynamics (CFD) simulations were performed in a transient regime to validate the model against observed data and to assess the potentiality of the two improvements. The system behavior is studied in terms of driving energy input and output energy production. Significant improvements of the system COP are observed (up to +20%). In the first case (water thermal storage), the overall COP is 4.1 during winter and 5.7 during summer, in the second case (PCM thermal storage), the COP is 4.1 and 5.9, respectively. The PCM thermal storage, in particular, is approximately 10 times smaller than the original design, and could be easily placed within the technical room.

A novel thermal storage strategy for CCHP system based on energy demands and state of storage tank

Under the thermal storage strategies in present studies, the power generation unit (PGU) in combined cooling, heating, and power (CCHP) system is turned on/off only based on the state of storage tank; or the operation state of PGU is determined by thermal demand or/and electric demand, which has no relationship with the state of storage tank. For these traditional strategies, either energy demands or state of storage tank is considered for the operation of system. This paper proposes a novel thermal storage strategy (TSS2) for CCHP systems, which determines the operational state of the PGU based upon electric demand, thermal demand and the state of thermal storage device. In this paper, the operation principle of TSS2 is described and compared with the traditional strategies. A hospital in Shanghai was used to evaluate and compared the performance of the system operating under different storage strategies. The results show that TSS2 improves the performance of CCHP system more significantly compared with the traditional strategies. TSS2 can make both PGU and capacity of thermal storage tank be fully used by CCHP system to produce more electric and thermal energy to meet demands of building. Compared with system without thermal storage, the minimum and maximum increase rates of annual total cost saving for traditional strategies are 0% and 14.84%, respectively. However, for TSS2, they can reach 3.79% and 20.22%. Similar conclusions could be achieved for CO2 emission reduction and primary energy saving. The effect of TSS2 on the CCHP system performance is different under different operation strategies.

Numerical simulation on heat transfer enhancement of phase change thermal storage devices for low-middle temperature

Using Ba(OH)2·8H2O as phase change material (PCM) and water as heat transfer fluid (HTF), we numerically simulated annular finned-tube heat exchangers. In order to measure and analyze the impact of parameters in the heating/cooling process, temperature changes of different monitoring points, fin widths, and fin pitches as key parameters were considered and applied. The experimental results show that the heat exchange process can be divided into three stages within a certain time. The faster heat transfer rate is associated with the greater temperature difference between PCM and HTF. Furthermore, fins width and pitch affect dramatically the heat charging/discharging rate. The large fins width or small fins pitch is beneficial for extending the heat exchange surface, leading to improve heat transfer efficiency.

Frequency Regulation by Distributed Secondary Loads on Islanded Wind-Powered Microgrids

Frequency regulation is critical to the successful operation of remote wind–diesel electrical grids. When the grid is in ‘wind–diesel’ mode, frequency regulation is (classically) the sole duty of the diesel electric generator (DEG). An alternative approach is proposed whereby responsibility for frequency regulation is shared by the DEG and a network of autonomous distributed secondary loads (DSLs) consisting of electric thermal storage (ETS) devices. This allows surplus wind to be distributed to residential consumers (as space heat) without the need for a centralized communication network. Numerical modeling of system dynamics with active DSLs is conducted using a SIMULINK wind–diesel hybrid test bed model. The effects of controller gain, installed capacity, switching time and unit coordination timing on frequency and voltage regulation is explored. It is shown that the DSLs can improve frequency regulation in wind–diesel mode while providing storable thermal energy to distributed consumers.

Running and economy performance analysis of ground source heat pump with thermal energy storage devices

This paper presents the thermal performance and economic analysis of a compound energy-supply system which composed of a ground source heat pump system (GSHPS) and a thermal energy storage system (TES). This system was simulated by TRNSYS and an engineering project was built for experiment, it’s found that the compound system could keep the soil in a better heat balance that is more suitable for discontinuous heating and cooling constructions. From the experiment process, it’s found the compound system has a higher COP than common GSHPS by 0.37 in winter and 0.04 in summer. For the price policy of off-peak electricity, the compound system could save 35.2% of the running cost in winter and 18.3% in summer than common GSHPS. In some case, the compound system could also has a lower initial investment.

Experimental investigation on bimetallic tube compositions for the use in latent heat thermal energy storage units

Based on the high energy density of phase change materials, latent heat thermal energy storage devices can play an important role in the future energy market. Therefore, the latent heat thermal energy storage technique is an interesting technology for industrial applications (e.g. batch processes) and power cycles. A key technology for such a storage device is the design of the heat exchanger tube, because the heat transfer rate by charging and discharging is the limiting factor based on the low thermal conductivity of the phase change material. The heat exchanger tube material used for such an application should have a high thermal conductivity and also a high mechanical resistance. Such a behavior can be found in a combination of different materials. The present paper deals with the design of such a heat exchanger tube composition consisting of a plain steel tube and an aluminum tube where fins can be attached. A novel bimetallic tube composition will be presented and compared with three common compositions. First, the mechanical stability of the bimetallic compositions was determined. Additionally a creep test of the used aluminum under operation conditions for a storage device using sodium nitrate as phase change material confirmed the utilizability for the operation in a latent heat thermal energy device.One of the main challenges for the compositions under investigation is based on the different thermal expansion coefficient for aluminum and steel, which results in different strain and creeping tendencies of the aluminum at operation temperature of the storage system, which is up to 340°C. A good heat transfer from the heat transfer fluid through the steel tube to the storage material around the fins can only be guaranteed through a close and stable connection between the two tubes. Compared to former solutions the fin circumference and the fin design are independent from the connection to the steel tube and allows individual arrangements of tubes and high packing densities.The experimental investigations have shown that the novel bimetallic tube composition is able to compensate these different strains and is capable to guarantee a stable connection between the steel and the aluminum tube. This high pressure and high temperature resistant bimetallic heat exchanger tube is easy to assemble and may play a key role for the development of thermal energy storages and other heat exchanging processes.

Two performance indices of TES apparatus: Comparison of MPCM slurry vs. stratified water storage tank

Microencapsulated phase change material (MPCM) slurry, which has a higher thermal storage capacity than water, was introduced as a storage medium in a coil-in-tank, but the key question is whether a competitive charging/discharging rate could be achieved. In this paper, two key indices, the volumetric thermal storage capacity at a given temperature difference and the charging/discharging rate variation over time, are newly defined. From the experimental results, it is estimated that the volumetric thermal storage capacity of the MPCM slurry is nearly twice that of water in the temperature range of 8–18°C. Finally, the overall heat transfer coefficient of the slurry storage device as a shell-tube heat exchanger was measured, and the external average convective heat transfer coefficient was also calculated. The two coefficients exhibited visible peaks during the phase change process with high-speed stirring and were much higher than that of water. However, the overall charging/discharging rates of the MPCM storage tank were observed to be much lower than the idealized stratified water storage tank (SWST), indicating that the design of an MPCM slurry thermal storage device needs to be further optimized.

Performance analysis of biofuel fired trigeneration systems with energy storage for remote households

Technical and economic modelling and performance analysis of biofuel fired trigeneration systems equipped with energy storage for remote households were carried out. To adapt the dynamic energy demand for electricity, heating and cooling, both electrical and thermal energy storage devices were integrated to balance larger load changes. The proposed systems were modelled and simulated by using the ECLIPSE process simulation package. Based on the results achieved, technical performance and emissions from the system had been examined. The impact of electrical and thermal energy storages was also investigated. Finally, an economic evaluation of the systems was performed. It was found that for a household, the internal combustion (IC) engine based trigeneration/combined heat and power (CHP) system is more suitable for heat to electricity ratio value below 1.5 and the biomass boiler and Stirling engine based system is beneficial for heat to electricity energy demand ratio lying between 3 and 3.4. Techno-economic analysis of the modelled trigeneration systems showed efficiencies of around 64–70% and Break-even Electricity Selling Prices of around £313/MWh to £357/MWh when fired by biofuels. Results also indicated that the economic viability of this type of trigeneration systems is significantly improved by the Renewable Heat Incentive (RHI) and Feed-In Tariffs schemes (FITs) by up to 46%.